The goal of this proposal is to understand the mechanisms underlying how negative endogenous signaling reduces chronic inflammation in rheumatoid arthritis (RA), which can provide novel therapeutic targets for diseases related to inflammation and tissue destruction. Current therapies for RA have serious negative effects on the patients? immune system, and can cause malignancies, pneumonia, and tuberculosis. Positive regulators of inflammation in RA (such as NF-?B) have been well characterized, however, the negative endogenous regulators of RA-induced inflammation and bone destruction, especially in macrophages and dendritic cells in RA pathological condition remain elusive. The proposed study will characterize the negative endogenous regulators of differentiation and activity in macrophages and dendritic cells (DCs) that attenuate RA-induced inflammation and tissue destruction. We noted that G?13 deficiency in macrophages or dendritic cells aggravated inflammation in mouse RA model. AAV mediated overexpression of local Gna13 constitutively active form (Gna13CA) not only significantly reduced bone destruction, but also dramatically inhibited inflammation in RA, indicating that Gna13CA could protect against inflammation in a mouse model of RA. Our RNA sequencing (RNA-seq) analysis showed that G?13 may negatively regulate macrophage and DC differentiation and activation and ChIP sequencing (ChIP-seq) analysis showed the enrichment of NF-?B motif site binding on the genome-wide promoters of the genes related to inflammation in G?13 deficient macrophages with TNF? or LPS stimulations. Our preliminary data showed that G?13 deficiency promotes Akt activity, STAT3 phosphorylation and NF-?B signaling pathway activation in dendritic cells and macrophages. Based on our preliminary studies, we hypothesize that G?13 is a master negative regulator that attenuates inflammation and bone destruction associated with rheumatoid arthritis through the G?13/RhoA/AKT/IKK/NF-?B pathway in macrophages and dendritic cells. Three specific aims are proposed to test our hypothesis. In Aim 1, we will determine the role of G?13 in inflammation and bone destruction in rheumatoid arthritis (RA) by characterizing the phenotypes and pathomechanism of conditional knockout (CKO) mouse models via loss-of-function approach in RA. In Aim 2, we will define the roles and therapeutic effects of G?13CA in inflammation and bone destruction in RA by characterizing the phenotypes and pathomechanism of conditional transgenic overexpression (OE) mouse models via gain-of-function approach in RA. We will characterize the molecular mechanism by which G?13 regulates inflammation and bone destruction in RA through G?13/ RhoA/AKT/IKK/NF-?B signaling, STAT3 signaling, and TLRs signaling pathways in macrophages and dendritic cells in Aim 3. The proposed study will provide important insights into the negative regulation of the cells of the immune system to effectively target inflammation and bone destruction in RA. Knowledge gained from this study may bring forth potential superior therapeutic means for treating RA and other inflammatory bone diseases.